Conventionally, for fixing devices for use in electrophotographic apparatus adopting electrophotographic processes such as copying machines, facsimiles, printers, etc., an internal heating process is generally adopted wherein a sheet having unfixed toner thereto is inserted into a contact portion (fixing section) between a fixing roller having stored therein a heater and a pressurizing roller which comes in tight contact with the fixing roller, and the toner is melted under an applied heat, thereby making the toner permanently affixed onto the sheet.
A schematic structure of the conventional fixing device adopting the described internal heating system is shown in FIG. 11. The fixing device is structured, for example, so as to include a fixing roller 101 having a silicone rubber layer 101b laminated on a core metal 101a made of aluminum and a pressurizing roller 102 having a silicone rubber layer 102b formed on a core metal 102a made of aluminum, wherein the pressurizing roller 102 is made in contact with the fixing roller 101.
The fixing roller 101 stores therein a heater lamp 103 as a heat source. The fixing roller 101 applies heat from the inside based on a result of detection by a thermistor 104 mounted on the surface of the fixing roller 101 to heat the surface of the fixing roller 101 to a predetermined temperature to be ready for a fixing operation (hereinafter referred to as a warm-up time).
On the other hand, the pressurizing roller 102 rotates while maintaining a contact with the fixing roller 101 during the warm-up time. Therefore, the pressurizing roller 102 is indirectly heated by the fixing roller 101. Therefore, the toner T on the sheet P as being transported to the contact portion between the fixing roller 101 and the pressuring roller 102 is heated by both the fixing roller 101 and the pressurizing roller 102, whereby the toner T is made permanently affixed onto the sheet P.
However, in the case of fixing the toner T onto the sheet P by the fixing device adopting the described internal heating method, for example, in the case of adopting toner which does not have desirable release properties such as color toner, etc., the toner may adhere to the surface of the fusing roller 101 (a so-called offset) in the fixing process. Namely, in order to improve color development or transparency of the color toner, it is required to set the fixing temperature high so as to apply a large amount of heat energy to the color toner formed on a multi-layer. However, when the temperature of the fixing roller 101 is increased so as to separate the toner T from the fixing roller 101 in a highly melted state, the adhesive exerted between the fixing roller 101 and the toner T exceeds the internal agglomeration force, and the toner T is parted in its inside, which results in an offset. On the other hand, when an attempt is made to prevent the high temperature offset by lowering the fixing temperature, the fixing strength on the interface between the toner T and the sheet P can be reduced, which results in insufficient fixing strength.
In order to counteract the described problem, especially, in the fixing device of the internal heating system adopting the full-color electrophotographic process, the fixing roller 101 is maintained at high temperature (for example, at around 180.degree. C.), and an offset-prevention-use oil 106 is applied onto the surface of the fixing roller 101 by means of an oil application mechanism 105 provided with a pair of oil application rollers 107, and after making the toner T affixed onto the sheet P, the sheet P is removed from the fixing roller 101 by a separation claw 108, whereby the sheet P having the toner T affixed thereon is removed from the fixing roller 101.
However, in the case of applying the offset-prevention-use oil 106 to the surface of the fixing roller 101, the following problems arise:
(1) A complicated structure is needed for uniformly applying the oil 106 to the fixing roller 101, which results in an increase in a cost for the fixing device; PA1 (2) The silicone rubber layer 101b of the fixing roller 101 deteriorates and swells with an application of the oil 106, which results in a shorter life of the fixing roller 101; PA1 (3) The device is contaminated by the leakage of the oil 106 which adversely affects other devices; PA1 (4) The oil 106 may adhere to the sheet P, which makes the user's hands dirty and makes him uncomfortable; PA1 (5) In the case of adopting the OHP to the sheet P, the oil 106 adheres onto the surface of the OHP, which results in poor transparency of the OHP is lowered; and PA1 (6) A periodical maintenance is required such as a supply of the oil 106, etc., and thus it is inconvenient to use. PA1 (i) obtaining beforehand a non-offset region defined by a critical border line obtained from a function of a fixing means surface temperature and a pressurizing means surface temperature in which a sufficient fixing strength for fixing toner onto the recording material can be ensured without generating a high temperature offset; and PA1 (ii) controlling the fixing means surface temperature and the pressurizing means surface temperature at the contact portion while the recording material is being transported therethrough to fall within the non-offset region. PA1 fixing means; PA1 pressurizing means for pressurizing a surface of the fixing means; PA1 heating means for heating the fixing means; PA1 first temperature detection means for detecting a surface temperature of the fixing means in a vicinity of the contact portion, the first temperature detection means being provided in a vicinity of the contact portion of the fixing means; PA1 second temperature detection means for detecting a surface temperature of the pressurizing means in a vicinity of the contact portion, the second detection means being provided in a vicinity of the contact portion of the pressurizing means; and PA1 heat application control means for controlling the heating means based on results of detection by the first temperature detection means and the second temperature detection means, PA1 wherein the heat application control means controls the fixing means surface temperature and the pressurizing means surface temperature at the contact portion while the recording material is being transported therethrough within a non-offset region in which a sufficient fixing strength for fixing toner onto the recording material can be ensured without generating a high temperature offset, the non-offset region being defined by a critical border line obtained from a function of a fixing means surface temperature and a pressurizing means surface temperature.
Moreover, in the conventional fixing device, the fixing roller 101 of a large heat capacity is heated by the heater lamp 103 (for example, 800 W) stored therein, and a heat is transferred from the surface of the fixing roller 101 to the pressurizing roller 102. As a result, the pressurizing roller 102 is also heated when heating the fixing roller 101. Namely, in the conventional fixing device of the internal heating system, most of heat applied to the fixing roller 101 is absorbed by the pressurizing roller 102. Therefore, in the described fixing device of the internal pressurizing system, a long warp-up time is required for heating the surface of the fixing roller 101 to temperatures at which the toner T can be fixed.